In recent years, optical materials have been actively studied. In particular, development of a material having high refractive index, low dispersibility (high Abbe number), and excellent heat resistance, transparency, easy moldability, low weight, chemical resistance, weather resistance, and scratch resistance is required in a lens material field.
A plastic lens is easily scratched, but is light, is unlikely to be cracked, and is easily processed, as compared with an inorganic material such as glass. Therefore, the use of the plastic lens spreads for not only a lens for glasses but also optical materials including a lens for a portable camera, a pickup lens and so on.
In a conventional optical member, the mechanical properties of a resin are improved by mixing an inorganic oxide filler such as silica with the resin (for example, see Patent Literature 1), and the scratch resistance is improved by layering a hard coat film containing inorganic oxide microparticles on a surface of the optical member (for example, see Patent Literature 2).
Due to a decrease in size of an electronic apparatus equipped with an optical member, a decrease in thickness of the lens is required, and an increase in refractive index of a material is required. The refractive index of the optical member is increased by mixing nano-level high-refractive index inorganic oxide microparticles of titanium oxide, zirconium oxide, or antimony pentoxide with a resin (for example, see Patent Literature 3).
However, during dispersion of nano-level inorganic oxide microparticles in an organic solvent, a large amount of dispersant is often contained (for example, see Patent Literature 4). In this case, bleed out during curing and coloring of a cured product may occur. This causes a decrease in refractive index. On the other hand, in a case where the dispersant is not contained or the amount of the dispersant is small, the dispersion stability of the inorganic oxide microparticles is decreased, the microparticles are aggregated in a dispersion, or a dispersion having a low solid content concentration of 15% by mass or less is only obtained (for example, see Patent Literature 5). When the microparticles are mixed with a resin and a cured product is produced, the cured product is easily clouded due to the aggregation of the microparticles, and the surface roughness is likely to be increased due to an aggregate. This causes problems in which scratch resistance and adhesion are likely to be decreased.
In order to improve the compatibility with the resin, various techniques for bonding an organic silane compound to a surface of the microparticles have been investigated. However, the surface of the microparticles is often made hydrophobic (for example, see Patent Literature 6). Therefore, a solvent in which the microparticles can be dispersed is restricted. When the surface of the microparticles is not sufficiently made hydrophobic, problems such as low compatibility with the resin and clouding of a cured film are caused.